To add some style to the notebook, Jupyter has the option to add a custom css file. This file should be located in ~/.jupyter/custom/custom.css. If this file does not exist yet, create the directory if needed and add the stylesheet. Add the following code and refresh the notebook to see if it works.
To make it easier to modify this file, create a link from the stylesheet to the working directory for the notebooks in order to be able to modify the stylesheet in the browser.
jitsejan@jjsvps:~/code/notebooks$ ln -s ~/.jupyter/custom/custom.css .